The first Islamic document to attack Christianity is not some polemicist in the 7th or 8th Century but the Quran itself. The Quran attacks doctrines such as the Trinity and the Crucifixion of Christ. It attacks Judaism as well.
The unfortunate thing for muslims is that the Quran misrepresents the two religions that it claims to supersede. Not just on some small technicality but very badly and on some of the core doctrines of both faiths.
A good example is Surah 9:30 which reads:
The Jews say, “Ezra is the son of Allah “; and the Christians say, “The Messiah is the son of Allah .” That is their statement from their mouths; they imitate the saying of those who disbelieved [before them]. May Allah destroy them; how are they deluded?
What is said about the Christians is entirely true. Christians have always taught that the Messiah is the Son of God. However, the statement that the Jews think Ezra is the Son of God is plain wrong. The Quran elsewhere knows that the nature of the Son of God in Christianity is a divine figure. Seeing as the terminology with Ezra and the Jews is used the same way as the Messiah and Christianity implies that Jews view Ezra the same way as Christians view Jesus which is entirely incorrect.
The problems continue for the Quran. Surah 5 attacks the doctrine of the Trinity but can’t get the Trinity correct. The Trinity is one God eternally existent in Three Divine Persons, the Father, the Son, and the Holy Spirit. The Quran thinks it is three Gods. These three God’s are Allah, Jesus, and Mary.
Surah 5:116 reads:
And [beware the Day] when Allah will say, “O Jesus, Son of Mary, did you say to the people, ‘Take me and my mother as deities besides Allah ?'” He will say, “Exalted are You! It was not for me to say that to which I have no right. If I had said it, You would have known it. You know what is within myself, and I do not know what is within Yourself. Indeed, it is You who is Knower of the unseen.
Christians have never said this so in theory, they have nothing to worry about. Regardless, the Quran thinks this is what Christians believe. This is apparent in the verses leading up to 116.
Verse 73 reads:
They have certainly disbelieved who say, ” Allah is the third of three.” And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment.
Since the word Trinity comes from the word three, the Trinity seems to be the object of attack here. Does this include Mary in place of the Holy Spirit? Yes it does, and we can tell this by verse 75 which reads:
The Messiah, son of Mary, was not but a messenger; [other] messengers have passed on before him. And his mother was a supporter of truth. They both used to eat food. Look how We make clear to them the signs; then look how they are deluded.
Mary is part of the three. The Quran is in error here regarding the Trinity. Though Christians venerate Mary, she is not part of the Trinity.
Imagine as a young boy, Muhammad goes on a caravan trip to Syria. No doubt, the towns and cities would have been full of Byzantine Churches. Byzantine Churches are full of icons. Most have large icons of the virgin Mary, the Mother of God. As an illiterate merchant, it would have been easy to look at the icon and reach some false conclusions. Years later when he heard that the Christians believe in a Trinity it is obvious that he concluded that the Christians worship three Gods, the Father, the Mother, and the Son.
When we look at it this way, we can see why the Quran made the error. We also see that the Quran is not from God since God knew what the Christians believed. He also knew what the Jews believed.
I would invite any Islamic apologist to make a serious effort to answering these serious objections to their faith.